1. Field of the Invention
The present invention relates to orthopedic bracing.
2. Description of the Related Art
Two functions that knee brace hinges perform are guiding the wearer's knee along a natural path of motion, and restraining the wearer's knee against unnatural and harmful motions. In order to accomplish these functions, many knee brace hinge designs attempt to mimic the natural motion of the knee. Some of these designs include monocentric hinges, polycentric hinges, four-bar-linkage hinges, and cam hinges.
The human knee, however, follows a rather complex path as it flexes and extends. In general, when viewed in the sagittal plane, the knee moves in a slide-and-glide fashion in which the femur partially rolls back posteriorly on the tibia as the knee flexes. Most of the motion of the knee occurs in the sagittal plane. However, a not insignificant amount of knee motion also occurs out of this two-dimensional perspective, as explained in detail below.
Many researchers have attempted to measure and model the three-dimensional motion of the knee. Based on this research, many designers have constructed knee brace hinges that approximate this three-dimensional motion. For example, U.S. Pat. Nos. 5,107,824, 5,611,774 and 5,792,086 describe three such hinges. These designs are all of the cam-slot type and include curved plates. The cam-slot arrangement allows for the hinge to undergo complex motion patterns and the curved plates allow for rotations to occur outside of the sagittal plane. However, these designs have a number of drawbacks.
First, cam-slot hinges require long cavities to be cut within the hinge plates. These cavities often result in large, bulky hinges, and leave little room for other features such as extension and flexion control stops. Second, curved hinge plates are costly to manufacture as compared to flat plates, which can often be made by inexpensive processes such as sheet metal stamping. Third, the curvature of the hinge plates in these designs is based upon the spacing between the medial and lateral hinges. This spacing changes with the size of the patient wearing the brace. Thus, to meet the needs of differently sized patients, a variety of sizes and shapes of curved parts must be manufactured. If the hinges are not properly sized and shaped, the hinges may not operate smoothly through their range of motion.
Recent research has revealed that the three-dimensional motion of the human knee can be described as a simultaneous rotation about two axes, each of which is fixed relative to the leg bones. (Churchill, Clinical Orthopaedics and Related Research, No. 356, pp. 111-118, 1998.). The first axis is the familiar flexion-extension axis. This axis is fixed relative to the femur, extends in the medial-lateral direction, and runs through the lateral and medial epicondyles of the femur bone. The second axis is the tibial rotation axis (also referred to as the longitudinal rotation axis). This axis is fixed relative to the tibia, runs parallel to the length of the tibia, and is located slightly medial to the center of the tibial plateau. In general, as the knee flexes the tibia simultaneously rotates internally about the tibial rotation axis.